The control of temperature is both necessary and commonplace in industry; moreover, temperature controllers are used abundantly to regulate temperatures across a broad range of applications from controlling the temperature of a general working area to heating and cooling specific devices to temperature extremes.
The type of temperature control can be any one of a multiplicity of types as required for each application. For example, controlling the general air temperature of a circuit board assembly area would permit the use of a temperature controller of a type which has less sensitivity and less accuracy than a temperature controller used for controlling the temperature of a highly temperature sensitive process such as the controlled diffusion of impurities into semiconductor wafers. The term "accuracy" as used herein refers to the maximum temperature variation of a body while its temperature is regulated by a temperature controller.
An example of a particular application for a temperature controller is the testing of an electronic device which is inherently temperature sensitive, e.g., a semiconductor device such as an injection laser for use in an optical communication system. Since the device being tested is temperature sensitive, it is necessary to have a temperature controller which is relatively accurate. It is also often desirable to test the device at each of a plurality of temperatures to provide information about the operating characteristics of the device across a specific temperature range. In such cases, it is desirable for reasons relating to the cost of testing to move from one test temperature to the next in as short of a period of time as possible. Thus, a temperature controller with a fast response time is advantageous for this application. The term "response time" is used herein to refer to the time taken to change the temperature of a body from a first temperature to a second temperature and stabilize at that temperature such that variations about the second temperature are within a prescribed limit.
Unfortunately, these two requirements, i.e., accuracy and fast response time, are generally incompatable in a conventional type of temperature controller. In a conventional temperature controller, the heating or cooling element is turned fully on until a temperature sensor indicates that a temperature related to the desired temperature has been reached. Common household thermostats typically operate in this manner.
The incompatability arises between the two requirements of response time and accuracy. A high powered heating or cooling element provides fast temperature transition but also produces large overshoots and undershoots around the desired temperature, also referred to as the set point. These overshoots and undershoots of temperature either actually slow down the response time by requiring a relatively long time for the temperature variations to fall within the prescribed limits, or force the test to be less accurate since the temperature of the device is varying within a relatively wide range. Conversely, a low powered heating or cooling element provides a relatively high degree of accuracy, but provides a relatively long temperature transition period.
Several methods have been used in the past to provide a temperature controller which is relatively accurate and has a relatively fast response time. Generally these types of controllers are electronic proportional controllers. Proportional controllers provide variable power levels of heating or cooling in response to the differential between the actual temperature and the desired temperature. A large temperature differential results in high power levels of heating or cooling and a small temperature differential results in low power levels of heating and cooling. A problem with proportional controllers is that they tend not to reach the desired temperature since power levels decrease to the point that a body stabilizes at a temperature other than the set point due to heat lost to or gained from the background environment. Variations to the basic proportional controller have been made, and such types as the proportional-integral (PI) and proportional-integral-differential (PID) have been developed to compensate for the shortcomings of the basic proportional temperature controller.
Another problem with proportional controllers, in addition to that mentioned above, is that the more sophisticated proportional controllers such as the PI and PID types are relatively complex.
Thus, it can be appreciated that a temperature controller which is relatively simple and yet provides relatively accurate temperature controls and relatively fast response times is highly desirable.